1. Field of the Invention
This invention relates to a mixing apparatus adapted for viscous fluid contents and a bottom ribbon blade used therein.
2. Discussion of the Background
Heretofore, helical ribbon impellers have been employed in many mixing apparatuses due to their better abilities to mix viscous fluid contents up and down and to transfer heat through vessel walls for temperature control in chemical reaction processes.
FIGS. 5A and 5B illustrate a mixing apparatus of a conventional type, which comprises a mixing vessel 1 having a cylindrical wall 2 and a bottom wall 3 of a semi-ellipsoid disposed at a lower end of the cylindrical wall 2, an impeller shaft 4, a plurality of horizontal struts 5 connected to the impeller shaft 4, a pair of helical ribbon blades 6 supported to an inner side of the cylindrical wall 2 by the horizontal struts 5, and a flat-shaped impeller blade 11 connected to a lower end of the impeller shaft 4, with its surface perpendicular to the bottom wall 3.
However, a drawback of the apparatus having the above arrangement resides in the fact that, since merely the flat-shaped impeller blade 11 is disposed in a lower region of the mixing vessel 1, the fluid contents in that region may be circulated merely in a circumferential direction by a rotating motion of the impeller blade 11, and consequently have poor mixing with the fluid contents in other regions of the mixing vessel 1, which may lead to problems in mixing operations.
To solve the above-noted drawback, two flat blades 12 are slantedly connected to both lower ends of the helical ribbon blades 6 and are extended in proximity with the center of the bottom wall 3 along an inner surface of the bottom wall 3, as illustrated in FIGS. 6A and 6B, or the helical ribbon blades 6 extend to the center of the bottom wall 3 as having substantially the same ribbon width and helical pitch thereof as illustrated in FIGS. 7A and 7B. The apparatuses of these arrangements solve the poor mixing of the fluid contents in a circumferential region of the bottom wall 3. However, there may be left a poor mixing region of substantial volume in a center region of the bottom wall 3.
Consequently, in the mixing vessel 1 having a relatively flat-shaped bottom wall 3, the poor mixing of the fluid contents has not been solved in a lower region, particularly in the center region of the bottom wall 3 merely by modifying the configuration of the helical ribbon blades 6 in conventional manners.
Contrary to the above arrangements, there has been proposed a conventional mixing apparatus of another type in view of the configuration of the bottom wall of the mixing vessel in order to solve the above-noted drawback. A mixing apparatus of this type comprises a circular-cone shaped bottom wall having an acute apex angle, and the helical ribbon blades extend towards the center of the bottom wall. However, in this arrangement, although a relatively large space for the installation is required due to the configuration of the bottom wall, the amount of the fluid contents treated in the mixing vessel does not substantially increase.
There has been further proposed a mixing apparatus described in the U.S. Pat. No. 3,675,902, wherein a bottom wall has a hemispherical shape, and a helical ribbon blade extends to the axial center of the mixing vessel, with its blade surface perpendicular to an inner surface of the hemispherical bottom wall. However, the configuration of the bottom wall presents a drawback in view of the installation space as described above. In addition, such configuration is not generally employed for mixing vessels except where the highly pressurized fluid contents are to be maintained in a vessel.
Accordingly, it is an object of the present invention to provide a mixing apparatus which can uniformly and effectively mix viscous fluid contents in a relatively short period of time without any regions of poor mixing of the fluid contents, in a mixing vessel having a relatively flat bottom wall of a semi-ellipsoid or dished shape.